1. Field of the Invention
The invention relates to a liquid crystal display device and method of manufacturing it, and more particularly, relates to a structure of a connection portion existing between a liquid crystal panel and a flexible substrate.
2. Description of the Related Art
There are various kinds of conventional liquid crystal display devices. In conventional liquid crystal display devices, by sealing a liquid crystal layer between two transparent substrates, and applying a voltage between the electrodes having fixed pattern formed inside of the substrates, it is possible to display various images.
In many cases, conventional liquid crystal display have a flexible substrate (referred to as an FPC hereinafter) to connect the liquid crystal display panel to control and drive circuits located outside of the liquid crystal display device. An example of the liquid crystal display device is described in the Utility Model (examined) Publication Hei 5-18789, (Japanese Utility Model (unexamined) Publication Hei 2-87473).
The connection portions have a predetermined pitch (namely the space as between adjacent wirings) adapted to the pitch of the external terminals, and the external terminals of the liquid crystal display device and the connection portions of the FPC are electrically connected to each other.
There are several methods to connect the FPC to the external terminals. One is to put the external terminals and the connection portion together and soldering them, the other is to place an anisotropic conductive film between the external terminals and the connection portions, then adhering them by applying heat and pressure.
In the latter method (heat seal method), the anisotropic conductive film is formed on the surface of the connection portion of the flexible substrate, and conductive particles are arranged at equal interval which are made of a couple of .mu.m fine metal particles, then so plating the connection portion of the flexible substrate so as to contact to the external terminals. These heated components are pressed together so that the anisotropic conductive film is crushed and the external terminals and the connection portions are electrically connected to each other through the conductive particles. As mentioned above, the FPC is adhered to the liquid crystal panel easily by means of the anisotropic conductive film.
FIGS. 7, 8, 9, and 10 show an example of conventional technology using the heat seal method. FIG. 7 is a perspective view showing a connection area for connecting the FPC to the liquid crystal display panel, and FIG. 8 shows an enlarged cross-sectional view of an edge area of the liquid crystal display device. Typically transparent electrodes 3 are formed inside of a top substrate 1, and transparent electrodes 4 are also formed inside of a bottom substrate 2. A liquid crystal layer 5 which is shown in FIG. 8 is sealed between top substrate 1 and bottom substrate 2, and a display area D is formed on the top substrate 1 as shown in FIG. 7. The bottom substrate extends outward beyond in comparison with the edge area of transparent substrate 1, and this extended portion forms an electrode lead-out area 2a. External terminals 3a, 4a are respectively connected to transparent electrodes 3, 4, which are transparent, formed in the shape of stripes and are arranged in parallel on the electrode lead-out area 2a.
On the other hand, in a resin sheet of a FPC 10, a plurality of conductive lines 12 are arranged almost in parallel, and the back side of the edge area of the conductive line 12 is exposed at a connection portion 10a of FPC 10. Connection portion 10a of FPC 10 is placed on the electrode lead-out area 2a as mentioned above and then heated and crimped. It is secured by the adhesive property of the anisotropic conductive film. As mentioned above, the external terminals 3a, 4a and the conductive lines 12 are electrically connected each other.
FIG. 9 depicts a printed circuit board 30 on which a liquid crystal drive circuit is mounted and connected to the liquid crystal display device through FPC 10. The back side of the conductive lines 12 are also exposed at a connection portion 10b on a circuit board 30, and the exposed area is also covered by the anisotropic conductive film. In the same way of above case, the connection portion 10b is secured to the printing circuit board 30, and the edge area of the conductive line 12 is conductivity connected to connection pads 31.
Conventional liquid crystal display devices have a number of disadvantages and problems as described below.
In the above mentioned conventional liquid crystal display device, when there is a transformation of the FPC, the stress caused by flexibility of the FPC may be applied to the conductive connection portion which is between the external terminals and connection portion. As a result wires may be cut, separated, or cause an interruption in the circuit. Therefore, reliability of such electrical connections of the conductive connection portion is reduced.
Particularly, in the above mentioned case, the thickness of the conductive layer on external terminals 3a, 4a is between 200 .ANG. and 600 .ANG.. This is a result of a process which the liquid crystal display device is manufactured, and this is much thinner than the thickness between 10 .mu.m or 40 .mu.m which is the thickness of the connection pads 30 formed on a printing circuit board 31. Therefore, in the case of crimping the anisotropic conductive film, the space between the FPC and peripheral portion of the external terminals is narrow, so the space where extra adhesive or/and conductive particles can escape is also narrow. Consequently, the contact area with the conductive particles in the anisotropic conductive film becomes much smaller than the connection portion of the connection pads 31. As a result of this configuration, the resistance of the connection portion tends to increase, and the risk of a break in the circuit also increases. To improve this point, there may be a way to form the anisotropic conductive film of connection portion 10a so as to be thinner than connection portion 10b and so on. However, in this way, it is difficult to maintain the quality of the connection portion, and there is a problem that the cost for manufacturing FPC 10 increase, therefore, the problem can not be solved in the above way.
In addition, in so called heat seal method for connecting the FPC through the anisotropic conductive film, although it is an easier operation for connecting, there is a problem, however, that due to the temperature, the conductive particles float in the anisotropic conductive film. This results in the conductive particles being separated from upper surface of the external terminals, and the conductive connection between them are then easily broken.
Finally, in conventional devices, there is a tendency during the manufacturing process not to have a perfect alignment of arranging conductive line 12 on external terminal 4a. FIG. 10 is an exploded view of a single connection of line 12 on external terminal 4a. As shown therein, conductive line 12 is misaligned on external terminal 4a by an amount .delta.. This results in a smaller area of electrical contact, thus making a poorer contact having a high resistivity.
With the advent, in recent years, of large capacity size of liquid crystal display devices, and the number of the external terminals has increased accordingly, thus resulting in the pitch between terminals being very fine (which is a particular with requirement of small-sized devices). As such there has been an increase in defective products.